This invention relates in general to data collection systems which use remotely located telemetry devices to transfer telemetry data from a remote site to a central processing location. More particularly, the invention relates to automatic meter reading (AMR) systems which use conventional subscriber telephone lines to transfer telemetry data, in the form of utility meter readings, from a customer's premises to a central processing location.
Prior to the existence of automatic meter reading (AMR) systems, the most common method for determining the amount of commodity delivered to a utility customer was to manually read a meter at, or in close proximity to, the consumer's premises. Because the utility meters were located at the point where the utility commodity was dispensed to the consumer, it became necessary for utility companies to establish routes where a "meter-reader" periodically visited each meter to record the amount of utility product consumed.
At present, many utilities, including gas, electric and water companies, continue to send meter-readers to consumer residences' to collect utility meter readings. However, there are practical limitations as to how often and how efficiently this procedure can be manually performed. For instance, weather and the ability to gain access to meters themselves (which are often inside the consumer's residence) directly impact the efficiency of this manual procedure. Today, where it is desirable for the utility to have almost instantaneous access to any meter, the manual method for collecting these readings is becoming both economically and operationally obsolete in view of the more sophisticated and reliable automatic techniques now available.
One very practical method for automating the process of collecting utility meter readings uses the existing telephone system and takes advantage of the already widespread availability of telephone service to both residential and business premises. Using this existing infrastructure, remotely located telemetry devices (at each consumer's premises) electronically upload utility meter readings as telemetry data to a central processing location via the subscriber telephone lines. This process is analogous to the procedure used by many PC users to electronically upload files by the use of a modem connected to the subscriber's telephone line, except that the AMR procedure is fully automatic. This invention relates to those AMR systems which utilize telephone line telemetry techniques.
In practice, the actual telemetry device used in an AMR system is called a meter interface unit (MIU). Located at the customer's premises, the MIU, as the name implies, is an interface between two different electrical environments. One side of the MIU, called the meter side, is connected to one or more utility meters while the remaining side, or line side, of the MIU is connected in parallel with, or across, the subscriber telephone line. In this regard, the telephone line side of the MIU is connected to the phone line in a manner similar to that used to add an additional extension phone, answering or FAX machine. Other than connecting the MIU to the subscriber line, no modification of the existing telephone line wiring is required.
In one particular type of AMR system, a real time clock within the MIU activates the device at a prescribed date and time. Once activated, the MIU seizes the phone line by taking the phone line off-hook, dials a preprogrammed telephone number to connect the MIU to a central processing station, reads the utility meters connected to it and then uploads the telemetry data from the MIU via the telephone line to the central station. Automatic meter reading systems which utilize this technique are known as dial-inbound systems, since the MIU dials into a central data processing center.
Since the telemetry transaction typically takes only a few seconds to complete, the MIU normally resides in a low power, standby condition (on hook) until such time as the programming of the MIU's real time clock causes the device to be activated again. Therefore, the MIU is said to be in a static mode between telemetry transactions and in a dynamic mode while engaged in the transfer of telemetry data across the telephone line.
An MIU may be viewed as an interface between two different electrical environments, namely the telephone line side and the meter side of the MIU. In order to protect both telephone company personnel and their counterparts in the utility industry from possible accidental electrocution, there is no DC continuity between the meter and telephone line sides of the MIU. Isolation is also required because the telephone subscriber loop must remain balanced, since translongitudinal imbalance would result in hum, noise and unacceptable loading of the telephone line. This isolation is obtained by using transformer (magnetic) coupling or optical coupling of signals on opposite sides of the isolation barrier.
With regard to the MIU itself, the terms meter and telephone line side more accurately describe how the telemetry device is electrically connected than how the MIU is operationally partitioned. For instance, because the meters attached to the MIU are typically read by a serial communication protocol similar to RS-232, a low power microprocessor is best suited to performing this task. Consequently, the same microprocessor may be used to format and transmit the telemetry data packet while simultaneously controlling the entire operation of the MIU. Therefore, functionally the meter side of the MIU is also called the control side.
Similarly, since it is desirable to have the MIU be self-powered from the phone line (with the exception of a small battery to sustain the real time clock in the static standby mode), the loop current from the central office switch provides the primary power source for the MIU. Consequently, the phone line side of the MIU is referred to as the primary side.
Because it is not acceptable for the MIU to disrupt or otherwise interfere with the normal operation of the subscriber telephone line, the MIU must include an "off-hook detector" or "line-status indicator" which is capable of detecting when the subscriber phone line is or is not in use. As noted before, the MIU is in either a static mode or a dynamic mode. As a result, the off-hook detector consists of not one function but two, namely a "dynamic off-hook detector" and a "static off-hook detector". The terms dynamic and static describe the current mode of the MIU.
Briefly, a "static off-hook detector" is employed when the MIU is in the static mode (standby state) to determine when the telephone subscriber commences use of the phone by lifting the telephone receiver or other telephone subscriber equipment seizes the phone line for communications use. In contrast, a "dynamic off-hook detector" is employed when the MIU is in the active state (MIU has seized the phone line and is actively transmitting data) to determine when the telephone subscriber commences use of the phone by lifting the telephone receiver or other telephone subscriber equipment seizes the phone line for communications use.
At first glance, the design of off-hook detectors may seem deceptively simple. However, getting them to function reliably, in practice, is a task requiring specialized design knowledge and skill. My patent, entitled Signal Processing Circuit For Use In Telemetry Devices, U.S. Pat. No. 5,202,916, the disclosure thereof being incorporated herein by reference, describes some of the complexities involved in designing a static off-hook detector for an MFG. Whereas the static off-hook detector serves to prevent the MIU from going to the dynamic (off-hook) mode while the subscriber line is in use, the dynamic off-hook detector permits the MIU to immediately disengage itself from the phone line should the subscriber attempt to use the telephone while the MIU is actively engaged in a telemetry transaction.
In practice, the dynamic off-hook detector is considerably more difficult to design than the static off-hook detector. The wide variation in operational parameters found within the telephone system complicates the task of designing a generic dynamic off-hook detector which functions reliably. Should the dynamic off-hook detector erroneously trigger during a telemetry session, the repeating malfunction may make it impossible to ever complete the telemetry transaction. In that case, the MIU would perpetually disengage itself from the line prior to the session's completion. At the other extreme, a dynamic off-hook detector which cannot detect a subscriber off-hook condition will not disengage itself from the telephone, thereby refusing to surrender up the phone line to the telephone subscriber, as it should. In fact, the task of designing this detector is so formidable that some MIU vendors do not even incorporate a dynamic off-hook detector function in their products. Since the functional requirements for static and dynamic off-hook detectors are different, each detector is customarily implemented as a separate device in conventional off-hook detectors.
One prior art dynamic off-hook detector approach is to sense the change in phone line loop current which occurs when the user takes a telephone device off hook. For example, the Schlumberger Model MIU T-3000 meter interface unit incorporates an analog to digital (A/D) converter for the sole purpose of periodically measuring the loop current on the primary side of the MIU. Although technically elegant, this is not a particularly cost effective implementation. Not only is the A/D converter expensive but additional components are required to transport the A/D's output (an 8-10 bit digital number representing the loop current) from the primary side of the telemetry device to the microprocessor on the control side of the isolation barrier. In addition, those skilled in the art will recognize that the environment under which line-powered telemetry devices must operate is one which is extremely power conscious. The A/D approach therefore has the added disadvantage that the A/D conversion consumes precious power resources.